Fix FindwxWidgets on Ubuntu and Debian
When the Ubuntu package libwxgtk3-dev is not installed, FindwxWidgets succeeds even the required libraries are not actually found. The reason for that is that wx-config does not perform any kind of check and uses hardcoded values.
This affects e.g. KiCAD build process and makes it fail with a confusing error message: https://bugs.launchpad.net/kicad/+bug/1630020
Solution is to check for presence of every header and library file obtained from wx-config.
Topic-rename: FindwxWidgets-verify-wx-config